1. Field of the Invention
The present invention relates to a steam injector for jetting highly pressurized water adapted to a boiler water supply particularly utilized for a water supply system in an emergency core cooling system such as light water reactor.
2. Discussion of the Background
A steam injector is generally utilized for a water supply system in a steam locomotive or a boiler of one type in which steam flows in its central region or another type in which water flows in its central region.
First, with reference to FIG. 25, one type of the steam injector in which the steam flows in its central region will be described. Namely, the steam injector shown in FIG. 25 has a casing 302 provided with a steam intake port 301, and a steam jetting nozzle 304 provided with a needle valve 303. The front, right hand as viewed, end of the steam jetting nozzle 304 is positioned near a water suction port 305. A steam-water mixing nozzle 306 and a pressure increasing diffuser 307 are arranged on a downstream side of the steam jetting nozzle 304, which are communicated with a discharge port 309 through a check valve 308. The steam-water mixing nozzle 306 is provided with a throat portion 310 to which an overflow discharge port 312 communicating with an overflow water duct 311 is opened, which is otherwise closed in accordance with an operation.
In the steam injector of the structure described above, when the needle valve 303 is drawn out from the steam jetting nozzle 304 by operation of a handle 313 connected to one end, i.e. the left hand end as viewed, of the needle valve 303 and the steam taken into from the steam intake port 301 is hence jetted from the steam jetting nozzle 304, the pressure at the water suction port 305 is made negative by the condensation of the steam to a value below an atmospheric pressure and the water is sucked from a tank or the like. The steam flows, while being condensed by a low-temperature water (less than 70.degree. C.) sucked from the water suction port 305, into the steam-water mixing nozzle 306 and then constitutes a downstream water flow at the throat portion 310.
Namely, because the enthalpy .eta..sub.g of the steam is higher than the enthalpy .eta..sub.1 of a saturated water by an amount corresponding to latent heat of evaporation, the latent heat evaporation is converted into a kinetic energy to thereby form a high velocity water flow. When this high velocity water flow passes the diffuser 307, the pressure is increased by an amount of .DELTA.P shown in the following equation in accordance with a hydrodynamic theory. EQU .DELTA.P=(1/2).rho..sub.w U.sub.t.sup.2
(.rho..sub.w =water density; and U.sub.t =flow velocity of high velocity water flow passing the throat portion) According to this equation, a discharge pressure higher than the steam supply pressure can be obtained by the steam injector. When the pressure on the outlet side of the diffuser 307 is sufficiently increased, the check valve 308 is automatically opened to thereby jet the pressurized water through the discharge port 309.
However, in the steam injector of the structure described above, only the discharge pressure of about 7 kg/cm.sup.2 G could be obtained, and such discharge pressure is a value which can merely be utilized for a boiler of a steam locomotive. It is considered that the cause of such limited low pressure increase resides in the fact that the longitudinal, i.e. axial, sectional area of the steam jetting nozzle 304 is made small or narrow towards the front end thereof.
Various attempts and studies have been carried out for increasing the discharge pressure utilized for the steam injector for an emergency core cooling system. FIG. 26 also shows a conventional example provided on the basis of these various attempts and studies.
The steam injector shown in FIG. 26 has substantially the identical structure to that of FIG. 25, but it is not provided with a needle valve such as that needle valve 303 in FIG. 25. Namely, the steam injector has a structure such as a diffuser having a gradually increased inner diameter towards the downstream side of the steam to thereby obtain a supersonic steam flow. A second nozzle is further located at the discharge side of the steam-water mixing nozzle 306 and the overflow discharge port 312 is formed on the upstream side of the throat portion 310. According to the steam injector of this structure, it is possible to obtain a discharge pressure of an amount about six or more times of the steam injector shown in FIG. 25.
As described above, in the steam injector, the steam is mixed with the low-temperature water to thereby condense the steam, the thus released latent heat of evaporation is converted into the kinetic energy and then into the pressure energy to obtain highly pressurized water. Accordingly, for the operation of the steam injector, it is necessary for the water to be supplied to have a temperature being sufficiently low to the extent capable of condensing the steam, and usually, the water has a temperature lower by about more than 70.degree. C. than the steam saturation temperature. For example, when the steam injector is operated in atmospheric pressure, it is necessary to use water having a temperature of less than 30.degree. C. because of the steam saturation temperature of 100.degree. C.
As is apparent from the structures of the steam injectors and the operational principles, it is desired to have a large temperature difference between the steam and the water at a time of being contacted with each other. However, in the described conventional structures, the heat of the steam is transferred to the water through the wall of the steam injection nozzle, so that the temperature of the water is made high in comparison with the water temperature of the water at the time of being supplied, thus the temperature difference is small. Furthermore, since the heat of the steam in the steam jetting nozzle is released, a portion of the steam is condensed, thus reducing its volume, resulting in lowering of the flow velocity of the steam. According to these reasons, the efficiency of the steam injector is itself reduced, and in an adverse case, the steam injector may stop operation.
Furthermore, in the steam injector which is not incorporated with the needle valve, there is provided a problem of causing pulsation of the discharge pressure variable in a short period. In the case of application of the steam injector to a nuclear power plant, the osccillation caused by the pressure pulsation may adversely affect the steam injector itself and the other equipment or lines, and therefore, it is required to reduce such pressure pulsation for ensuring stable operation of the nuclear power plant.
Since the pressure pulsation of the steam injecter is caused by the fact that the steam is not stably condensed, it is necessary for the reduction of the pressure pulsation to facilitate the condensation of the steam and to carry out continuous reaction. In order to achieve this purpose, it is considered to be effective to increase the contacting area between the steam and the water. The contacting area between the steam and the water may be determined by the hydraulic equivalent diameter of the front end of the water nozzle. The hydraulic equivalent diameter corresponds to a value obtained by dividing the cross sectional area of the water nozzle port by the wetted perimeter length, and the contacting area can be increased by making this value small.
However, since the the cross sectional area is determined by the capacity of the steam injector, in the conventional round-type nozzle in which the wetted perimeter length naturally corresponds to the peripheral length of the water nozzle port, the cross sectional area is also naturally determined. Accordingly, it may be said that the increasing of the contacting area between the steam flow and the water flow has a restricted limit.
FIGS. 27 and 28 further show other examples of the steam injectors of the prior art each in which the water flows through the central region of the steam injector. FIG. 27 represents a horizontal type and FIG. 28 represents a vertical type, but of these steam injectors have basically similar structures. That is, in the steam injector shown in FIG. 28, a water nozzle 316 is incorporated in a body 315 connected to the casing 302 and a needle valve 303 is inserted into the water nozzle 316, wherein the pressure of the steam is increased together with a steam from an adjacent steam suction port by a steam-water mixing nozle 306 disposed on the downstream side of the water nozzle 316. The steam injector shown in FIG. 28 has substantially the same structure as that of FIG. 27 but it is not provided with the needle valve.
In the case where the conventional steam injectors are utlized as emergency water supply systems, the operation condition and the pressure are deemed as variable factors which balance conditions on the water supply side, so that it is necessary for the injector side to reach a rated pressure as soon as possible and to maintain a stable operation for a long time. Furthermore, it is desirable to control the startup characteristic from the operation free from a complicated control system. Moreover, in the case of the steam injector being utilized as a fluid driving source, it is necessary for the steam injector to keep stable the jetting condition.
In the conventional structure of the steam injector, there is a case in which the jetting condition of the steam injector reaches the rated power in a certain time interval just after the operation of the steam injector and the jetting pressure lowers as the time passes thereafter. This is considered to be based on the deformation between the steam nozzle and the mixing nozzle due to temperature variation and pressure variation on the periods of the waiting condition and the operating condition. Accordingly, suppression of such deformation will result in improvement of the operational characteristics.
Although adjustments of the flow rate and the pressure may be varied with the location of the needle valve, the performance of the steam injector is significantly affected by the positional relationship between the steam nozzle and the steam-water mixing nozzle and it is hence necessary to keep this positional relationship most suitable. However, in the conventional steam injectors, the operating temperatures differ from each other since at the starting time they are at a normal temperature and at during operation they are at a high temperature. This temperature difference results in the change of the positional relationship, which adversely affects on the originally expected performance.
Furthermore, in the conventional steam injectors in each of which the needle valve is provided, and the needle valve is shifted to adjust and change the flow area of the water supply nozzle to attain the optimum dischrge power, the flow areas of the steam are rapidly contracted at the steam jetting nozzle portion, thereby causing the supersonic steam flow. For this reason, there may be caused wear, due to the supersonic steam flow, to the outer wall surface of the water supply nozzle forming the steam jetting nozzle portion and to the inner wall surface of the casing of the steam injector, and furthermore, there is caused erosion of an area of the wall surface of the throat portion positioned downstream side of the steam jetting nozzle portion by the high velocity water flow, thus causing the wear to this portion.
As described, when the wear to the respective wall portions progresses, the flow area itself changes, and hence, the balance of the flow rates of the water and the steam changes gradually, resulting in degradation of the performance of the steam injector With respect to the steam-water mixing nozzle, it becomes difficult to ensure a stable condensation of the steam.
These problems are also made significant for the water supply device of an emergency core cooling system of a power plant, for example, which requires high reliability and performance.